The high cost and increased environmental footprint of fossil fuels and limited petroleum reserves in the world have increased the interest in renewable fuel sources. Renewable resources include ethanol from corn and sugar for use in automobiles, and plant oils for use as diesel fuel. Research in the diesel fuel area includes two main areas, bio-diesel and green diesel.
Transesterification of fatty acids in triglycerides into methyl esters using methanol and a catalyst such as sodium methylate produces FAME (Fatty Acid Methyl Ester), which is commonly referred to as bio-diesel. These methyl esters, mainly linear C14 to C22 carboxylic acids, can be used as fuel or can be blended into diesel refined from crude oil sources. The transesterification reaction is complex. To be used as diesel fuel, costly modification of diesel engine is necessary as well as conversion of associated piping and injector configurations. Other disadvantages include poor performance of bio-diesel in cold weather applications, limiting its world wide use to warmer climates, and poor emissions. In addition, use of bio-diesel increases maintenance costs due to poor lubricity, increased viscosity, and high oxygen content. Bio-diesel, while a renewable resource, brings a high cost of use for processing and use in engines.
Diesel from renewable resources, commonly referred to as green diesel, involves converting the fatty acids in triglycerides into linear alkanes via hydrodeoxygenation (HDO). The triglyceride backbone is converted to propane and separated. Green diesel can be used as a fuel by itself or as a mixture with diesel from petroleum feedstocks (petro diesel) with little to no engine modification and can be processed in refineries currently refining crude oils. Current processes involve multiple steps to obtain green diesel fuel with comparable properties with petro diesel. Steps include hydrodeoxygenation, hydroisomerization and/or hydrocracking.
Delmon, B. “Catalysts in Petroleum Refining 1989” in: Studies in Surface Science and Catalysis, Eds. Trimm, D. L., Akashah, S., Absi-Halabi, M., and Bishara, A. (Elsevier, Amsterdam, 1990), pp 1-38, discloses the transformation of a very large portion of crude oil to usable products depends on cracking and hydrotreating processes. Over the last several decades, hydrotreating processes have become more complex and diversified and includes such processes as, hydropurification (e.g., removal of sulfur, nitrogen, oxygen, metals, etc.), hydroconversion (e.g., production of jet fuels or lubricants), and hydrocracking (mild or heavy hydrocracking). Specifically, the removal of sulfur, nitrogen, oxygen, and metals are called hydrodesulfurization, hydrodenitrogenation, hydrodeoxygenation, and hydrodemetallization, respectively.
Certain hydrotreating catalysts for use with petroleum feedstocks comprise one or more non-precious metals such as nickel, cobalt, molybdenum and tungsten supported on mono- or mixed-metal oxides such as alumina, silica or silica-alumina. The catalysts can be promoted by Group I metals (e.g., lithium, sodium and potassium) and/or fluorine, boron, and phosphorus. The catalyst is activated by simultaneous reduction and sulfidation in place before subjecting it to hydrotreating reactions. Catalysts consisting of molybdenum supported on alpha-alumina with promoters such as cobalt (Co—Mo/Al2O3) or nickel (Ni—Mo/Al2O3) are extensively used in the hydrotreating of petroleum fractions and resids.
The catalyst most commonly used for the production of diesel from renewable resources comprises a precious metal such as platinum and/or palladium. Murzin et al. in Industrial Engineering Chemical Research, Vol. 45 (2006) pp. 5708-5715, disclose numerous metals used for such catalysis. Platinum and palladium gave the best conversion of desired products. Nickel catalysts produced unwanted heavier products such as dimers due to the recombination of moieties resulting from extensive cracking of the feed material.
Processes are known to produce green diesel. Such processes suffer from one or more deficiencies such as requiring multiple steps, multiple reactors, different catalysts per step and expensive precious metal catalysts.
While there have been great efforts in the green diesel research area, there remains a need for a process of hydrotreating of a renewable feed source where the hydrodeoxygenation, hydroisomerization and hydrocracking processes are simplified using a less expensive, non-precious metal catalyst.